Many machinery components having a round bar shape are subjected to a hardening process. When the component is symmetrical about a center axis, it is usually rotated about the axis during quenching to prevent warping. If the workpiece is asymmetrical about its center axis, such as a round bar component having a flat side extending, therealong, it tends to warp due to inconsistency in the cooling rate. To prevent warping during quenching of such an axially asymmetrical component, a press quenching method which provides constrained quenching is commonly utilized. In the press quenching method, all or a part of a workpiece is securely held at a pressure in a set of dies having inside walls adapted to match and is accommodate the shape of the workpiece, and cooled down by thermal dissipation throughout the dies or direct by immersion into a cooling liquid. In the case of thermal dissipation throughout the dies, the cooling of the workpiece is effected through the dies which are directly cooled by liquid.
Recently, tubular shaped components tend to be adopted used in place of the traditional solid bars in machinery especially in automobiles, for minimizing the overall weight of the products. However, in the conventional press quenching method, the tubular workpiece is easily collapsed due to it hollow being and being softened by heat.
The press quenching method hardly allows the cooling speed to be controlled in desired locations. The quenching strain on the workpiece is a combination of a thermal strain and a phase transformation strain which is varied depending on the phase transformation. Consequently the quenching strain does not depend on only the cooling rate, but also depends on the chemical composition of the material. Nevertheless, if the cooling rate is locally controlled, the warping of the asymmetrical workpiece can be suppressed to a certain degree in an early stage of the warping. Moreover, if cooling rate is changed locally, the resultant hardness of a target region of the workpiece will be adjusted in combination with the heating temperature to almost a desired value so as to provide a favorable material characteristic.